(AP) OKLAHOMA CITY - Five people were critically injured Saturday when their single-engine aircraft crashed in far northwestern Oklahoma City.

The aircraft had taken off from Wiley Post Airport and was scheduled to fly to Enid Woodring Regional Airport when it may have experienced engine trouble and attempted to return to Wiley Post, said Roland Herwig, spokesman for the Federal Aviation Administration.

The plane clipped a tree before it came down in a ditch in front a Bank of Oklahoma branch along Northwest Expressway at 8:52 a.m., authorities said.

"It appeared to me that it was trying to make an emergency landing. He kept it upright the whole time," said Oklahoma City fire Battalion Chief Steven Lumry. "He landed in the best orientation he could for a crash or emergency situation."

The aircraft's occupants, two men and three women, were taken to the University of Oklahoma Medical Center and Deaconess Hospital in critical condition, said EMSA Spokeswoman Lara O'Leary. They were not immediately identified.

"Our paramedics were driving down Northwest Expressway and they saw the single-engine plane crash," O'Leary said. "It was fortuitous that they were there immediately."

The plane, a 1976 Beech Bonanza, was destroyed in the crash, Herwig said. No one on the ground was injured, but the hazardous materials unit was called to clean up when fuel began to leak into a nearby storm drain.

"They were able to mitigate the circumstance rather quickly," Lumry said.

"I could see the firemen kind of panicking in front of the plane and trying to get the people out of the plane," said Amber Dowell, who works at Tan & Tone America across the street. "It was kind of crazy and scary... I've never seen anything like it."

The westbound lanes of Northwest Expressway were shut down while officials investigated the crash.

The cause of the crash and the reason for the plane's return to Wiley Post return are under investigation, Herwig said. He said the National Transportation Safety Board also plans an investigation.